Automatic control system



Aug i8, w53 F, w. MREDATH CONTROL SYSTEM AUTOMATIC 2 Sheets-Sheet I.

Filed Aug.

Aug l i953 F. w. MEREDITH AUTOMATIC CONTROL SYSTEM Filed Aug.

latentecl Aug. 198, 119593 AUTOMATIC. CONTROL SYSTEM Frederick William Meredith, London, England, assigner to. S. Smith & Sons (England)A Limited', LOndomEngland, a British company Application August 9, 1948, Serial No. 43,336' In Great Britain August` 5, 1947 16 Claims. E.

rFhis invention relates to automatic control systems for maintaining acondition. atl a predetermined value by supplying an appropriate signal to a servo system controlling the condition. It is usual in such control systems to provide a detector for detecting deviation of the condition from the predetermined value and a generator for generating the signal proportional to the deviation. This signal is applied tothe servo system and causes a servornotor to operate so as to restore-the conditionV to the predetermined value. Control systems of this type will be referred to in this specification and accompanying claims as an automatic control system of the type described.

Other control signals, such as signals proportional to the rate of deviation, servomotor displacement and servomotor speed, may also be applied to the servomotor, but the algebraic sumY of all the signals applied to thev servomotor should be zero when the condition is steady and at a predetermined value. Circumstances may however arise in which this sum is not Zero. For instance, there may be a torque demand on the motor or one of the signal generators mayV have a zer-o, er- :i:'-

ror. Furthermore, it may only be possible to provide the signal necessary to meet the torque demand or to balance the false signal from the generator having a zero error by a signal from the generator operated by the deviation detector and this can only be achieved by the steady state condition differing from the predetermined value. The present invention is directed towards overcoming this difficulty.

According to the present invention, an automatic control system of the type described comprises means for giving the deviation detector or the generator controlled thereby a zero error to compensate for any Zero error in the system so that the condition is stabilsed at the required predetermined value.

The invention also consists in an automatic control system of the type described, which system comprises a deviation detector giving a mechanical indication of the deviation of the condition from a predetermined value, a signal generator mechanically coupled to said detector-and means for displacing the part of the coupling connected to the generator relative to the part connected to the detector, whereby, if any zero error exists in the system so that the condition is stabilised at a value other than the predetermined value, displacement between the two parts of the coupling may be effected until the condition is stabilised at the predetermined value.

2. Thus: the detector and generator may be coupled together through. a clutch whereby, if any zero exists in* the system. so-that the. condition is stabiiised at a value other than theV predetermined value, the generator may be. declutched from the detector and reclutched with the condition at the predeterminedvalueand the generator emitting a signal compensating for said zero error.A Means should,- also beprovided for ensuring that thek signaly from the generator does notvary. during such time as the clutch is. disengaged. As an alternative toy or in addition to the clutch, the de.- tector and generator may be coupled together throughl a differential gearV which provides means for varying the relative. positions ofv detector: and

generator.

Where thecontrol' system is. electrical, the gen'- era-tor preferably comprises an electrical; potentiometer the moving part thereof being coupled to.. the detector;

rhe automatic control system ofthe invention may be applied to an air-craft in order to stabilise thev aircraft in a predetermined attitude, In thiscase the detector is preferably a gyroscope arranged. tofgive a mechanical indication of. the deviation ot the aircraft. from itspredetermined attituder about oneA of the controlY axes. of the aircrait. If the axis concerned isthe yaw axis, thev gyroscope may. conveniently be of the type monitored, to maintain a iixed orientation with respect to the. horizontal component. of the earths magnetic eld, as disclosed for instance in, British patent specification No. 585,506.

The nature of this invention. and the manner in which it is to be performed will be more clearly understood from the following description of an aircraft. automaticpilot, reference being made to the accompanying drawings which illustrate a modication inVv the system described in patent application Serial No. 655,684, now Patent No. 2,607,550 and in which:

Figure 1 illustrates the system for controlling an-aircraftv about-the yaw axes,

Figure 2 shows in further detail the rate of turn. instrument shown in-Figure 1:,

Figure 31 illustrates in further detail the sideslip detector shown in Figure l,

Figure el' illustrates in further detail the tangent potentiometer shownin Figure 1,

Figure 5: illustrates in further detail the differential gearfand the clutch shown in Figure 1.

As'shownin Figure-1, the aircraft is stabilised about'theyaw axis by means of arateoflyaw gyroscope i, mounted on a platformv Il gimballe'd about the pitch axis |21' in a` gimbalfring I3 pvoted in the aircraft about the roll axis I4. The gimbal ring I3 can be rotated relative to the aircraft about the roll axis Iii by means of a handle 5i) through gearing 5l, the support 52 for the roll axis ii being secured to the aircraft. During straight rlight conditions, the platform is centrally positioned about the roll axis.

The rudder 2 is coupled through a clutch Il and gear box i8 to an electric hysteresis motor I9 of the kind described in Britain patent specification Nos. 576,248 and 576,249. Motor I9 is wound for two-phase operation, one phase (a reference phase) being coupled to an A. C. source 20, and the other phase the control phase, being coupled to the output of an amplier 2i. Motor i9 is coupled to a hysteresis generator 22 of the kind described in British patent specification No. 576,351. The generator is also wound for two-phase operation, one phase being connected to the source 2t so that an output proportional to the speed of the generator and therefore of the motor is generated in the other phase, the phase of the generator output being substantially in quadrature with that of source 2t.

ln order to bank the aircraft, the gimbal ring i3 is rotated about axis Effi by handle i6 and relay 3i is energised. By a servo system, which is not illustrated and which forms no part of the present invention, the ailerons are operated to bank the aircraft so as to maintain the platform level in space i. e. the aircraft is banked through an angle equal and opposite to that through which the gimbal-ring i3 is rotated.

Before considering the operation of the rudder control circuit it is necessary to say a word about the components in the circuit. The rate gyroscope I will be described in greater detail hereinafter, and it will suice for an understanding of Figure l to state that a voltage is produced across its output terminals which is proportional to the rate of turn and is of the same frequency as that of the source but in quadrature therewith lagging or leading according as the turn is in one direction or the other. Tangent potentiometer 2d is mounted on the axis ibi. This instrument will be described in greater detail hereinafter and it will suffice for an understanding of Figure l to state that a voltage appears across the output terminals of the potentiometer 2li which is proportional to the tangent of the angle through which the platform i i is turned in roll from its normal central position by the handle 5e and is of the same frequency as that of the source 2@ but in quadrature therewith lagging or leading according as the turn is in one direction or the other. A pendulum 25 which will be described with reference to Figure 3 is provided to detect side slip.

In addition to these components there is an instrument for giving an A. C. signal in accordance with. the deviation of the craft in azimuth from a preset course. This instrument co nprises a directional gyroscope 3 coupled through a differential gear Ii to a driving clutch member The corresponding driven clutch member It drives through shaft It? the wiper arm It@ of a potentiometer IiI having an arcuate resistance It. A source M3 having an A. C. cutput of the same frequency as that of source and in quadrature therewith is connected across potentiometer resistance IG@ and an output is taken between the mid-point il@ of resistance lo@ and the slider arm It. It will be appreciated that the output will be an A. C. signal proportional to the displacement of the arm ,It

from centre point II@ and of the `same frequency as source 20 but in quadrature therewith lagging or leading thereon as the displacement is in one direction or the other. Engagement of the clutch members HG2, ID3 is effected by a solenoid W5. Solenoid I is de-energised to disengage the clutch when the gimbal ring I3 is rotated in roll and whenever the aircraft is being flown under manual control. When the clutch members IGZ, 03 are engaged the shaft im may be rotated relative to the directional gyroscope by rotation of the control knob IUS associated with the diiferential gear IM.

The side slip pendulum may be short circuited by closing the pair of contacts 33. The relay 3I controls these contacts so that when the gimbal ring i3 is rotated about the roll axis from the horizontal the contacts 33 are open. During straight iiight the contacts are closed.

Considering the circuit during level flight of the aircraft the output from the rate of yaw gyroscope I is applied in series with the outputs from potentiometer lili and feed-back. generator 22 to the input of amplifier 2l. On commencing the level flight the directional gyroscope 3 is clutched to the potentiometer arm I so that when the aircraft is ying on a predetermined course the arm Itis is coincident with the centre point il@ of potentiometer resistance IUS and hence no signal is emitted from potentiometer. The sum of three voltages is thus applied to the input of the amplifier 2l. These voltages are proportional respectively to the rate of yaw as measured by rate of yaw gyroscope l, the deviation-froin the preset course as measured by gyroscope 3 and potentiometer IiI and the speed of the motor i9. Potentiometer IOI acts purely as a monitor, that is, it gives a comparatively weak signal compared with those derived from rate of yaw gyroscope I and generator 22 and its effect on the operation of the motor I9 during a disturbance may be neglected. The motor I 9 operates to reduce the input to the amplifier and since the gain of the amplifier is made Very large it will reduce this input substantially to zero that is to say during a disturbance the motor will run at such a speed that the output of the generator 22 is substantially equal but opposite to the output of the rate of yaw device I, or in other words the motor E9 and therefore the rudder 2 will be operated at a speed proportional to the rate of yaw. This will have the effect of very rapidly stablising any disturbance in yaw. If after a disturbance has been stabilised the aircraft is flying oi course, the wiper arm iiB will be displaced from centre point iIJ and a signal will be applied to yaw amplier 2i which will cause the craft to return to the desired course.

Normally when the aircraft is stabilised on course there are no signals from the rate of yaw gyroscope l, potentiometer iti, or yaw generator Z2. lit may be however', that the gyroscope I or the generator 22 has a zero error so causing signals Yto be generated when the aircraft is not turning or when the generator 22 is not turning. .Also7 out of trim conditions may result in a torque demand on the rudder motor IS to hold the craft on course, which will necessitate a non-zero input to the amplier lwhen the craft is on course, and the amplifier itself may have a aero error, i. e. zero input may not correspond to zero output. In such circumstances it becomes necessary for the potentiometer I iii to generate a signal either to balance the false signal from the rate of turn acnegtor.

gyroscope I-I or generator 22, tov supply the torque demandor to compensate fbr the amplifier" zero error. This the potentiometer ISUI.` can.- d'o* if the aircraft turns on to a different course. Consequently the air-criaitunderv these conditions would be stabili'sed oft course ais zero7 signal from the potentiometer would not correspond? to the: desired course of the craft.

If it is found that the aircraft isflying off course for any of the above reasons, two courses f action are opentothe pilot; He; maydisengage the automatic control systemy aridbring the aircraft on tothe appropriate course by manual control. Since potentiometer ISIS is provided with no centering spring the clutch member it? will remain in the position at which'` it was left before disengagement was effected, i'. e; at that position in which the signalfrom the potentiometer l0! cancels out the aero errors or supplies asuficient signal to amplifier 2l to enable motor i9 to generate the torque necessitated by an out of trim condition. However, as the aircraft is turned in azimut-h by manuali control the clutch member ESS will rotateA relative toclutch member i532. When the aircraft is brought onto the corrected course the clutchr members m2, i are engaged. The aircraft will then iiy on the corrected course with the slider arm |08 displaced from centre point IIB by a sufficient extent to cancelv zero error in the system' or to cause motor le to exert the necessary torque to counteract the out of trim condition.

The pilot may also bring the aircraft on the correct course vby rotating control knob IDG. Rotation of this knob-produces rotation or the clutch members |32, ill-3 and hence of slider lSB relative to the directional gyroscope (which maintains its orientation in space substantially constant). Thus by the rotation of the knob in the appropriate direction and by an appropriate amount 11' the aircraft can be made to iiy on the desired course the slider arm ISS being. displaced from centre-point i Iii by a suiiicient amount to provide the required signalas before.

Change of course is effected by bank turns. To achieve this the gimbal ring I3 is rotated by handle S to bank the aircraft as previously den scribed. When the gimbal ring I3 is. rotated in roll, relay Si and solenoid I are actuated to disengage clutch members IEE., I-GS- and to open contacts 'When the gimbal ring I3 has been rotated through the required angle of bank the tangent potentiometer Efe will emit anA. C. signal proportionate to the tangent of this bank. This signal is applied in series with those from rate of yaw gyroscope I and sideslip detector to the input of ampliiier 2l thereI being no signal from potentiometer ic i. The rudder servo system will operate until the signals from the rate of roll gyroscope I and tangent potentiometer 24 are equal and opposite, i. e. the aircraft will turn in azimuth at a rate appropriate to the setting of the tangent potentiometer. As is wellknown, for a correctly banked turn the rate of turn in azimuth is directly proportionalto the tangent of the angle of bank and inversely proportional to the rair speed. The constant of proportionality of the tangent potentiometer 24 is set to give a signal equal to or opposite to that. given by the rate of turn instrument I when the aircraft is turning at the correct rateof travel at the cruising speed. If the aircraft is not travelling at its cruisingv speed, side slip will' take place and the side slip detector 25 will give a sig-nal causing` the rateof turn in azimuthto-be varied so as toicounsteractt'he side-slip..

When the aircraft; has. turned on to the new course, the giinbal. ring I3fis returned to: its original position andi the: relay 31|. d'e-energised. and solenoid i225 re-energisedr.. The bank. is;v thus taken oit" and1 further turning stopped.. During the turn the clutch member IlllA rotates relative to clutch member Ii2; so:` that at the end of the turn when the clutch is 11e-engaged, the directional gyroscope 3f andl potentiometer' I=IlIf will monitor the aircraft` on the new course; In ad?- dition, if before the turn the conditions existed in which a signal' from thel potentiometer III-I wasrequired in order to maintain the aircraft on a correct coursel when theclutch is 14e-engaged the aircraft is flying on its new course the signal fromI the potentiometer liti. will again be that required to compensate for any zero error inthe system;

En patent application Serial No. 655,684' a suitable systern for control of the aircraft about the pitch and roll axes is described.

i suitable rate of: turn gyroscopes: for use as gyroscope I, is illustrated diagrammaticallyf in Figure 2. As there shown a gyroscope comprises a rotor 3c mounted on spin axis 5'- in a gi'mba'l ring gimballed about an axis 31 on a xedbase Carriedby the gimbal ring Misa-potentiometer contact 39'- arranged to sweep across-a xed arcuate potentiometer resistance 4U asf the ring rotates relatively to the base about the axis 37. The ring is restrained by a spring 4I- toa central position in which the contact 39 engages the mid point 132 of the resistance` at'. Theresistance is connected across the A. C'. source 43". The electrical output of the gyroscope appears between the contact 39 and the mid point 42T.

In operation any turn about an axis normal to the base `38v results in. precession. of the gyro scope about the axis 311 against the spring restraint to an extent determined by the rate of turn. Hence the output voltage will. be pro.- portional to the rate of turn.

As an alternative to the use of gyroscopes in the arrangement described withY referenceto Figure-1', the rate of turn devices described in paent' application Serial No. 5.043072. may be used.

A pendulum.. suitable for. usev as side-slip detector 25 is illustrated diagrammatcally inY Figure 3. As there shown a pendulum: llt of suitable conducting materialv is pivoted toswing about an axis parallel to the roll. axis. The pendulum 44 sweeps across an arcuate resistance: 45 connected to the A. C. source d3, so that the electrical output appears between the pendulum 44 and the mid point 4S of'the resistance d'5.

The tangent potentiometer 2% is illustrated diagrammatically in Figure 4. As there shown, an arm of conducting material is attached to the axis I4 so as to rotate with the gimbal. ring I3 and sweeps over a linear resis-tancev 48 connected across the A. C. source 43. The output appearing between the arm 4'! and the mid point 49 of the resistance is thus proportional to the tanget of the angle through which the gimbal ring I3 is turned.

Figure 5 illustrates the dierential gear LSA, clutch members I t2, H33 andpotentiometer IiIII in more detail. The slider arm me of potentiometer IIlI is secured to shaft IIE'I.- which. at' its other end isl secured to clutch member |02, and clutch member HB3` shown as disengaged from clutch member |02' can be rotated easily about shaft I2` which iscoaxiall with shaft ID-l'f. Glu-tch member ID3 has attached to it the gear Wheel I I I engaging with planet wheel I I2 mounted on a gear Wheel IIS which can rotate freely about shaft |20. Shaft |20 is driven by the directional gyroscope 3 through gear-wheel II8. Gear wheel II4 located between gear wheel II3 and clutch member |03 is keyed to shaft IZU and also engages with planet wheel II2. Gear wheels III and IIA are of equal diameter but wheel III has one more tooth than wheel IIJ. Control knob IBS is attached to a further shaft H5. Accidental rotation of shaft II5 is prevented by the friction action of spring l I6. Gear wheel II'I on shaft H5 engages with gear wheel II3. The solenoid controlling the clutch member H32 is illustrated at |05.

Rotation of shaft is transferred to shaft |01 through gear wheel IUI, planet Wheel II2 gear Wheel III and clutch members |02, m3, the position of planet wheel I I2 being normally fixed. When the clutch members IIJ2, E33 are engaged and control knob |016 is rotated, gear Wheel II3 is rotated causing planet wheel I i2 to turn about shaft Il and hence turn gear wheel III, gear wheel IIA being held stationary by the directional gyroscope. It will be seen that the gearratio between shafts H5 and I Eil is very loW, so that a -ne adjustment of the potentiometer slider with respect to the directional gyroscope by rotation of knot Ii is obtainable.

I claim:

1. An automatic control system for maintaining a condition at a predetermined value comprising a detector giving a mechanical indication of the deviation of the condition from the predetermined value, a signal generator mechanically coupled to said detector through a clutch and giving a signal in accordance with said deviation, a servo system controlling the condition and controlled by said signal, means for effecting engagement and disengagement of the clutch, means for ensuring that the signal from the generator does not vary upon disengagement of said clutch and during such time as the clutch is disengaged and means for causing relative movement of the engaging members of said clutch when disengaged, whereby, if any zero error exists in the system so that the condition is stabilised at a value other than the predetermined value, the generator may be declutched from the detector and reclutched with the condition at the predetermined value and the generator emitting a signal compensating for said zero error.

2. An automatic control System for maintaining a condition at a predetermined value comprising a detector giving a mechanical indication of the deviation of the condition from a predetermined value, a signal generator giving a signal in accordance with said deviation mechanically coupled through a clutch and a differential gear which provides means for varying the relative positions of the detector and the generator, means for effecting engagement and disengagement of the engaging members of said clutch,

means for ensuring that the signal from the generator does not vary upon disengagement of said clutch and during such time as the clutch is disengaged, means for causing relative movement of said members when disengaged and a servo system controlling the condition and controlled by said signal whereby, if any zero error exists in the system so that the condition is stabilised at a value other than the predetermined value, the relative positions of the generator and detector may be varied so that the generator emits a signal compensating for said zero error with the condition at the predetermined value.

3. An automatic control system for maintaining a condition at a predetermined value comprising a detector giving a mechanical indication of the deviation of the condition from a predetermined value, an electrical potentiometer the moving part thereof being mechanically coupled through a clutch to said detector, giving an electric signal in accordance with said deviation, an electric servo system controlling the condition and controlled by said signal, means for effecting engagement and disengagement of the clutch means for ensuring that the signal does not vary upon disengagement of said clutch and during such time as said clutch is disengaged, and means for causing relative movement of the engaging members of said clutch when disengaged whereby if any Zero error exists in the system so that the condition is stabilised at a Value other than the predetermined value, the potentiometer may be declutched from the detector and reclutched with the condition at the predetermined value and the potentiometer emitting a signal compensating for said zero error.

4. An automatic control system as claimed in claim 2 in which the servo system is electrical and the said generator comprises an electrical potentiometer, the moving part thereof being coupled through said clutch and differential gear to said detector.

5. An automatic control system for maintaining an aircraft in a predetermined attitude about one of .the axes of the aircraft comprising a detector for detecting deviation of the aircraft about said axis, said detector giving a mechanical indication of said deviation, a signal generator generating a signal in accordance with the displacement from a datum position of a moving part thereof couping means to couple said detector and said moving part, a servomotor system controlled by said signal and controlling .the appropriate control surface of the aircraft, means for effecting engagement and disengagement of the engaging members of said coupling means, means for ensuring that the signal does not vary upon disengagement of said clutch and during such time as said clutch is disengaged and means for causing relative movement of said members when disengaged whereby, if any zero error exists in the system so that the aircraft is stabilised at an attitude other than ythe predetermined attitude, the generator may be declutched from the detector and reclutched with the aircraft in the predetermined attitude and the generator emitting a signal compensating for said zero error.

6. An automatic control system as claimed in claim 5 in which said generator is an electrical potentiometer and said motor control system is electrical.

7. An automatic control system as claimed in claim 6 in which the generator and detector are also coupled through a differential gear providing means for Varying relatively the positions of the generator and detector when .the clutch is engaged.

8. An automatic control system as claimed in claim '7 in which the coupling means is disengaged when the aircraft is own under manual control.

9. An automatic control system as claimed in claim 8 wherein the coupling means comprises electromagnetic means for coupling and decoupling said detector.

10. An automatic control system as claimed in claim 9 in which the detector is a gyroscope.

11. An automatic control system for maintaining a condition at a predetermined value comprising a detector giving a mechanical indication of the deviation of the condition from the predetermined value, a signal generator mechanically coupled to said detector through a .clutch and giving a signal in accordance with said deviation, means for ensuring .that the signal from the generator does not vary upon dis'- engagernent of said clutch and during such time as said clutch is disengaged, whereby if any zero error exists in the system so that the condition is stabilised at a value other than the predetermined value the generator may be declutched from the detector and reclutched with the condition at the predetermined value and the generator emitting a signal compensating for said zero error.

l2. In combination in an automatic control system for maintaining a condition at a desired value, a directional gyro arranged to detect deviations of said condition from said value, a potentiometer having a movable arm, drive means having a clutch connecting said arm and said gyro and connected to move said arm in response to said gyro, said arm being constructed to remain stationary during disengagement of said clutch means and indicate zero error in the system upon return of said condition to said value whereby said condition can be stabilized at a value representing the algebraic sum oi' said desired value and any zero error.

13. In combination in an automatic control system for maintaining a condition at a desired value, means to control said condition, detecting means to detect departure of said condition from said value, signal means connected to said control means, and connecting means for connecting and disconnecting said detecting means and said signal means and serving to maintain the signal therefrom constant for a limited time While dS- connected at the zero error value of the system, said connecting means and said signal means serving to compensate for any zero error in the system by reconnection after disconnection whereby said signal means continually produces said maintained signal containing said zero error and introduces it into the system by said reconnecton.

14. An automatic pilot control system for maintaining a craft on a desired course, in combination, a movable craft, force exerting means on said craft to control said craft with respect to said course, detecting means to detect departure of said craft from said course, rate of turn sensitive means mounted on said craft, sideslip indi'- cating means mounted on said craft, operating means for said force exerting means, signal means connected to said departure detecting means, said operating means connected to each said signal means, said rate of turn sensitive means and said side-slip indicating means and integrating the signals therefrom and variable connecting means for connecting and disconnecting said signal means and said departure detecting means and serving to cause said Signal means to signal to said operating means detected departures of said craft from said course and any zero error in the system whereby said craft can be stabilized on a selected course and any zero error compensated by connecting, disconnecting and reconnecting said connecting means.

15. The combination set forth in claim 14, said variable connecting means having two positions, for one said position said signal means produces a signal determined by said departure detecting means, for the other said position said signal means produces a constant signal equal to a Drevious value determined by said departure detecting means.

16. In combination in an automatic pilot control system for maintaining a movable craft on a desired course, a movable craft, means to control said craft with respect to said course, detecting means to detect departure of said craft from said course, signal means connected to said control means and two-position variable connecting means joining said detecting means and said Signal means, in one position of said connecting means said signal means producing a signal determined by said detecting means and in another position of said connecting means said signal means producing a constant signal equal to the last previous value detected by said detecting means, said signal means thus being caused to signal to said control means detected departures from said course and any Zero error in the system whereby said craft can be stabilized on a selected course and any Zero error compensated.

FREDERICK WILLIAM MEREDI'IH.

References Cited in the file of this patent UNITED STATES PATENTS Number Name Date 1,581,147 Whitaker Apr. 20, 1926 1,998,939 Mittag Apr. 23, 1935 2,361,790 Noxon Oct. 31, 1944 2,415,429 Kellogg et al. Feb. 11, 1947 2,451,263 Webb Oct. 12, 1948 2,464,629 Young Mar. 15, 1949 

